离子渗ZrN与喷丸复合对TC4合金疲劳行为的影响

为改善钛合金耐磨和抗疲劳性能,采用离子渗技术在TC4钛合金表面先渗Zr,再渗N形成梯度ZrN合金化层,并与喷丸强化(SP)后处理复合。采用扫描电镜(SEM)、X射线衍射仪和显微硬度计分析测试了改性层的组织结构和硬度沿层深的分布,探讨了改性层对TC4钛合金疲劳性能的影响规律和作用机制。结果表明,TC4钛合金离子渗层由ZrN-TiN复合相表层和富氮的Zr-Ti固溶体次表层组成,表面以ZrN相为主,其硬度较基材提高了3.2倍,合理强度的喷丸后处理未造成渗ZrN层表面明显损伤,并使表层硬度进一步增大,硬度沿层深呈现梯度递减分布。渗ZrN层明显降低了TC4合金的疲劳抗力,此归于ZrN-TiN层韧性低、表面粗糙度大。SP后处理则使渗ZrN的TC4钛合金疲劳抗力显著提高,不仅远高于TC4钛合金基材,而且远大于SP处理状态,原因归于复合处理引入了数值高、分布深的残余压应力场,并使表面粗糙度降低、组织改善,有效抑制了疲劳裂纹的萌生和扩展。

Effects of combined plasma zirconium nitride metallurgy and shot peening on fatigue behavior of TC4 titanium alloy

In order to improve wear and fatigue resistance of titanium alloy, plasma alloying technique was applied to treat TC4 alloy with zirconium followed by nitrogen, and the gradient metallurgical zirconium nitride (ZrN) layer was fabricated. The surface metallurgical layer was then treated by shot peening. The microstructure, chemical composition and microhardness profile of the modified surface layer were then evaluated by using scanning electron microscope, X-ray diffraction, glow-discharge photo-electron spectroscopy and micro- hardness tester. The fatigue behavior and mechanism influencing by the modified surface layer were investigated. The results demonstrate that the uppermost modified layer is composed of ZrN-TiN compound phase (mainly ZrN) and underlying nitrogen-rich Zr-Ti solid solution, and the surface hardness is raised 320%. The modified surface layer dramatically reduces the fatigue resistance of base material, due to the low toughness of ZrN-TiN layer and the increase of surface roughness. After combined with shot peening, the fatigue resistance of TC4 alloy is improved significantly, more than the specimens of both base material and shot peening alone. Since shot peening introduces a deep and high value compressive residual stress into the surface, lowers the roughness and refines the surface microstructure of TC4 titanium, and thus the initiation and early growth of fatigue cracks are restrained.